Optical fibers used for optical communications are installed in offices, homes, and the like, or are used in optical fiber cables, in which a plurality of optical fibers is disposed in a sheath. In the case in which optical fibers are installed as described above, the optical fibers are typically disposed with at least a part of them being bent. In the case in which optical fibers are used in optical fiber cables as described above, the optical fibers are disposed being bent in the overall structure, because the optical fibers are typically spirally disposed in the sheath.
It is known that when light is propagated through a core of a bent optical fiber, a so-called bending loss occurs, in which light is leaked from the core to cause optical losses. As one example of optical fibers that reduce such bending losses, an optical fiber is known, in which the relative refractive index difference between a core and a cladding is increased. Patent Literature 1 below describes such an optical fiber. With such an optical fiber, the optical confinement power of the core is increased, and hence light is not easily leaked from the core, compared with the case in which the relative refractive index difference between the core and the cladding is small, allowing bending losses to be reduced as well. As another example of optical fibers that reduce bending losses, an optical fiber is known, in which a low-refractive index layer whose refractive index is lower than the refractive index of the cladding is provided around the core. Patent Literature 2 below describes such an optical fiber. This optical fiber is sometimes referred to as a trench optical fiber, because the low-refractive index layer is in a trench shape from the viewpoint of the refractive index. With such an optical fiber, light is not easily leaked from the core, because the low-refractive index layer confines light, allowing bending losses to be reduced as well.    [Patent Literature 1] Japanese Patent No. 4268115    [Patent Literature 2] JP2013-88818 A